This invention relates to a method for determining the nitrogen concentration in single crystal silicon, and more particularly, to a method of using low temperature Fourier Transform infrared spectroscopy (LT-FTIR) in the far infrared spectral range (FIR) to determine the concentration of nitrogen in single crystal silicon produced in accordance with the Czochralski method.
The properties of nitrogen in silicon have become a matter of interest in recent years for several reasons. For example, nitrogen is known to lock, or pin, crystal dislocations, it is know to impart strength and warp resistance to silicon with low oxygen concentrations, and it is known to form shallow thermal donors (STDs) with an ionization energy of 35-37 meV in as-grown and annealed silicon. In view of the important role nitrogen plays in silicon, several measurement techniques have been applied to study nitrogen in silicon, including infrared spectroscopy, luminescence, electron paramagnetic resonance, Hall effect, deep level transient spectroscopy, secondary ion mass spectroscopy (SIMS), and nuclear reaction analysis.
In addition to studying the effects of nitrogen in silicon, some of the foregoing methods have been used to determine the quantity of nitrogen in silicon. For example, SIMS has been used to determine the concentration of nitrogen in silicon. See R. S. Hockett and D. B. Sams, Electrochemical Society Proceeding, Volume 2000-17, p. 584. The detection limit for nitrogen in silicon using SIMS is about 1-2xc3x971014 atoms/cm3, and is limited primarily by background nitrogen (i.e., the residual nitrogen in the chamber atmosphere after evacuation).
Fourier Transform infrared spectroscopy (FTIR) has also been used to determine the concentration of nitrogen in silicon. The presence of nitrogen in silicon gives rise to several IR absorption peaks. The peaks of interest for these FTIR nitrogen measurement methods occur within the medium infrared region (MIR) between about 600 and about 1000 cmxe2x88x921. Specifically, the two main nitrogen-related MIR absorption peaks are at about 963 and about 766 cmxe2x88x921 and are associated with the vibrational modes of the molecular nitrogen species (Nxe2x80x94N). The most widely accepted of determining nitrogen concentration by FTIR uses the absorption peak at 963 cmxe2x88x921 for which a calibration coefficient of 1.83xc3x971017 cmxe2x88x922 has been determined. See Y. Itoh and T. Nozaki, Appl. Phys. Lett. 47, p. 488 (1985). The nitrogen concentration is determined by the equation:
[N](atoms/cm3)=1.83xc3x971017xc3x97xcex1(963 cmxe2x88x921).xe2x80x83xe2x80x83(1) 
The absorption peak at 766 cmxe2x88x921 has also been used and the calibration coefficient of is 4.45xc3x971016 cmxe2x88x922. See P Wagner et al., Apply Phys. A 46, p. 73 (1988); and Watanabe et al., Semiconductor Silicon, p. 126 (1981). The nitrogen concentration is determined by the equation:
[N](atoms/cm3)=4.45xc3x971014xc3x97xcex1(766 cmxe2x88x921).xe2x80x83xe2x80x83(2) 
The intensity of the nitrogen-related absorption bands, or peaks, in the MIR is always low for a silicon comprising about 1015-1016 atoms/cm3 (the maximum absorbance is 0.1), and much lower than oxygen and carbon peaks in the MIR. Thus, the accuracy of determining nitrogen according to these methods is less than desirable especially at low nitrogen concentrations. Additionally, these methods are even less accurate for determining the nitrogen concentration in CZ silicon than in FZ silicon. This is because the calibration coefficient for 963 cmxe2x88x921 absorption peak was determined using float zone-grown (FZ) single crystal silicon in which the vast majority of the nitrogen is in molecular form with the small remaining fraction being primarily atomic. In contrast, in CZ silicon, a significant fraction of the nitrogen is in the form of Nxe2x80x94O complexes, and as a result, using the foregoing method underestimates the total nitrogen concentration in CZ silicon.
Thus, a need continues to exist for a method of quickly and accurately determining the concentration of nitrogen in CZ silicon, especially at low concentration levels such as below about 1xc3x971014 atoms/cm3.
Among the features of the invention, therefore, is the provision of a method for quantitatively measuring nitrogen in Czochralski silicon based on the detection of one or more Nxe2x80x94O complexes by means of low temperature Fourier Transform infrared spectroscopy (LT-FTIR) in the far infrared spectral range (FIR); the provision of a method for quickly and accurately determining the concentration of nitrogen in single crystal silicon; and the provision of a method for detecting nitrogen at concentrations below about 1xc3x971014 atoms/cm3.
Briefly, therefore, the present invention is directed to a method for measuring a concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature Ta for an annealing time ta to saturate the silicon sample with nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
The present invention is also directed to a method for measuring a concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature Ta for an annealing time ta to saturate the silicon sample with nitrogen-oxygen complexes, wherein Ta is selected as a function of a detection limit for the nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
Additionally, the present invention is directed to a method for detecting a measurement of concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature Ta for an annealing time ta to saturate the silicon sample with nitrogen-oxygen complexes, wherein Ta is selected as a function of annealing time available to saturate the silicon sample with nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
In another embodiment, the present invention is directed to a method for detecting a measurement of concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature Ta for an annealing time ta to saturate the silicon sample with nitrogen-oxygen complexes, wherein ta is selected as a function of a detection limit for the nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
Other objects will be in part apparent and in part pointed out hereinafter.